This invention relates to a process for crosslinking polypropylene polymers. The process employs steps of irradiation and annealing. The invention further relates to crosslinked polypropylene polymers produced by the process.
There have been earlier proposals to crosslink polypropylene polymers, by processes which employ ionizing radiation. However, there is an inherent difficulty as, in parallel with crosslinking, the backbone polymer chains are prone to degradation by ionizing radiation, this degradation being known as chain scission. The overall effect can be a substantial degradation in mechanical properties.
In Japanese Patent Application No. 74 23582 of Japan Atomic Energy Research Institute, referred to in Chemical Abstract 58944V, vol. 82, 1975, page 66, there is disclosed the crosslinking of polyolefins, using ionizing radiation in the presence of both acetylene and a chloride-containing monomer such as vinyl chloride. The process takes place at room temperature, at at least 1 atm. (10.sup.5 Pa) pressure. The radiation level is 10.sup.8 -10.sup.9 Rad., a very high level of radiation.
In U.S. Pat. No. 3,835,004 of Japan Atomic Energy Research Institute there is disclosed the crosslinking of polyolefins including polypropylene, using .gamma.-radiation in the presence of acetylene or 1,3-butadiene, and of a fluorine-containing monomer, for example tetrafluoroethylene. Use of the two crosslinking agents is said to produce polyolefins with better properties than those crosslinked with acetylene or 1,3-butadiene alone. In Example 4 of U.S. Pat. No. 3,835,004 the process as applied to high density polypropylene pellets, using both acetylene and tetrafluoroethylene, and 4.8 Mrads of .gamma.-radiation, gave a product with a gel fraction of 0.536. In comparison, use of acetylene alone gave a gel fraction of 0.474.
In U.S. Pat. No. 4,134,812 of Japan Atomic Energy Research Institute and The Furukawa Electric Co., Ltd., there is disclosed a method of manufacturing shaped articles, by blending a polyolefin with crosslinking agents, moulding to the desired shape, and crosslinking the shaped articles by irradiation, using an electron beam. The crosslinking agents, used together, are tetrakis-[methylene-(3,5-di-t-butyl-4-hydroxy-hydrocinnamate)]-methane and a compound X-Y.sub.m where X is a group free from acetylenic linkage, Y is a group containing acetylenic linkage and m is an integer of 1 or more. Preferred compounds X-Y.sub.m are dipropargyl maleate and dipropargyl succinate. Acetylene gas is said not to be possible for use, "because it is not only little miscible with a poly-.varies.-olefin due to its own gaseous condition, but also gives rise to the formation of an explosive product by reaction with heavy metals". There is one example of the invention as applied to polypropylene, Example 5. In this example a moulded polymer sheet 3 mm thick, containing 100 parts polypropylene, 5 parts tetramethylene dipropiolate and 0.5 parts of the tetrakis compound, was irradiated in the open air at a dose rate of 20 Mrads. The gel fraction of the irradiated product was 0.75.
Experiments on the irradiation of polypropylene using .gamma.-radiation are described in European Polymer Journal, 1979, vol. 15, pp. 379 to 387 (W. K. Busfield and J. H. O'Donnell). Different isotactic polypropylene film samples were irradiated in vacuo at 35.degree. C. at a range of doses, from 5 Mrads to 150 Mrads. After irradiation the samples were stored for more than 3 weeks before being opened to the atmosphere. There was a significant decrease in the tensile breaking stress and strain, attributed by the authors to fairly extensive chain scission.
Experiments on the crosslinking enhancement in polypropylene film by .gamma.-irradiation in the presence of acetylene are described in British Polymer Journal, 1986, vol. 18, No. 5 (W. K. Busfield and R. W. Appleby). Quenched and pre-annealed samples of polypropylene film were irradiated using .gamma.-radiation, at a dose rate from 5 to 40 Mrads, at 35.degree. C., in the absence of air and the presence of acetylene gas. The samples were annealed for one hour at 100.degree. C. in order to remove any remaining free radicals before opening to the atmosphere. The resultant polypropylene films have improved mechanical properties, in particular improved creep resistance, especially in pre-annealed samples.
Similar experiments are described in Polymer Communications, Feburary 1986, vol. 27, pp. 45-46 (R. K. Appleby and W. K. Busfield). Isotactic drawn polypropylene film was irradiated with .gamma.-radiation at dose rates of 5, 10 and 15 Mrads in ampoules containing acetylene gas. Following irradiation the ampoules were annealed for 1 hour at 100.degree. C. prior to opening. Polypropylene samples having good creep resistance and gel contents of 0.61 (5 Mrads), 0.74 (10 Mrads) and 0.77 (15 Mrads) were obtained.